1. Field of the Invention
The present invention relates generally to a cathode ray tube (hereinafter, abbreviated as CRT) for color televisions or high-definition monitor televisions. Specifically, this invention relates to a field emission electron source that can be used as a cathode of an electron gun used in a high-luminance CRT requiring high current density, or an electron gun used in, for example, an electron beam exposure device utilizing converged electron beams.
2. Related Background Art
In recent years, with the introduction of thin-type displays such as a liquid crystal display, a plasma display and the like, the market of flat panel displays has been expanding rapidly. Even with such a trend, for home-use television sets with a size of about 32-inch diagonal, displays using CRTs still have been in the highest demand in terms of their prices and performances. Furthermore, terrestrial digital broadcasting is planned to be newly introduced at full scale starting from the year 2003, and therefore it is expected that there will be a drastic change in the technologies of television displays. With televisions and their surroundings making a transition to a digital system, displays have been required strongly to have high resolution performance. However, there has arisen a possibility that such requirement might not be met satisfactorily by conventional television technologies that have been in wide use.
In a CRT, an electron gun is used as a core portion for displaying images, and the performance of the electron gun strongly affects resolution performance. When a cathode used in an electron gun has increased current density, an effective cathode area can be reduced, thereby allowing the resolution performance to be improved. Thermal cathode materials currently used as cathodes have undergone various technological improvements so as to have improved current density. The thermal cathode materials are now approaching their physical limitations and are reaching a difficult situation where no further remarkable improvements in current density can be made. In recent years, a cathode for electron guns used for digital broadcasting has been going into practical use. Such a cathode has been requested to have an improved current density by a factor of about 6 to 10 than of a conventional thermal cathode.
Meanwhile, a conception regarding the use of a cold cathode in an electron gun has been proposed conventionally. A cold cathode has an inherent advantage of its high current density, and thus conventionally, some products of the cold cathode such as an electron microscope have become commercially practical.
As a first example of a cold cathode used in a CRT, JP48(1973)-90467 A discloses a conception regarding a color picture tube using a field emission cathode. In addition to the above-described advantage of the high current density, when used in a color picture tube, a field emission cathode can provide an advantage in its suitability for achieving lower power consumption. When a conventional thermal cathode is used, the use of a heater is required for electron emission. This entails a standby power consumption of about several watts on a constant basis even during a period in which the electron gun is not operated. In contrast to this, when a field emission cathode is used, a heater is not required at all. This can provide not only the advantage of suppressing unnecessary power consumption but also the advantage of allowing an electron gun to be started instantaneously.
FIG. 6 is a cross sectional view of a field emission electron source disclosed in JP48(1973)-90467 A. Cathodes 61R, 61G and 61B provided on a substrate 60 are formed of three pieces or three groups of conical cathode projections 62. The cathodes 61R, 61G and 61B are insulated from each other by an insulation layer 63, and supplied with luminance signals for red (R), green (G) and blue (B), respectively. A gate electrode 64 formed of a thin metallic film with apertures corresponding to the respective cathode projections 62 is supplied with an appropriate potential such that a predetermined field emission current is generated from each of the cathode projections 62 when the luminance signals are applied to the cathodes 61R, 61G and 61B, respectively. A control electrode 66 is disposed on the gate electrode 64 through an insulation layer 65. After being transmitted through the control electrode 66, an emitted electron beam travels in the same path as that in a conventional electron gun to be focused on a screen. This configuration has enabled a high current density operation that conventional cathodes have failed to realize and allowed high luminance and high resolution properties to be attained.
However, with the future spread of digital high-definition television broadcasting, there will be an increasing demand for CRTs that can achieve a resolution not less than two times as high as that in the case of conventional broadcasting. Generally, because of an operation principle of the CRTs, the closer to a peripheral portion of a screen a beam spot of an electron beam is, and the higher the luminance of the screen is, the more distorted in shape the beam spot is. This causes the resolution of the screen to be deteriorated. For the improvement in resolution of a CRT, this can be addressed effectively, for example, by further improving the CRT in current density. However, because of a close relationship between the current density of a cathode and the life of the cathode, an increase in current density causes the life to be decreased. Thus, from a practical viewpoint, there is a limit to the degree to which the current density can be increased.
Generally, as materials of cold cathode elements, metals having high melting points such as molybdenum and the like often are used in such cold cathodes. Further, because of a constraint of a manufacturing process, a CRT tube as a finished product manufactured by the CRT manufacturing process has a vacuum of about 10xe2x88x925 Pa in the tube. When a cold cathode was operated at a current density of about 10 A/cm2 under the condition of this vacuum, the following problem arose. That is, in the tube of the CRT, there exist molecules of various kinds of residual gases generated in the manufacturing process and an electron beam emitted from the cold cathode collides with those molecules, so that a large number of ions were generated. Our evaluation test has revealed that these ions thus generated are accelerated and collide against a surface of a cold cathode element of the cold cathode, so that the cold cathode element is damaged, and thus a beam current emitted from the cathode is lowered substantially. When the cold cathode is applied to an electron gun for televisions, this lowering of a beam current emitted from the cold cathode results in a decrease in luminance of a CRT screen. Accordingly, a desired luminance cannot be maintained during long-term use, which is a serious problem.
A mechanism of this ion generation is considered to be such that an electron beam accelerated and converged to about 100 eV by an electric field in the vicinity of the cold cathode collides with a gas molecule remaining near the cold cathode. An amount of ions to be generated is proportional to a vacuum level and a current density. Therefore, in order to suppress the ion generation, a method to be selected has been an improvement of vacuum level or a decrease of the current density. Because of a constraint of the CRT manufacturing process, it is difficult to improve the vacuum level considerably. Also, the degree of increasing the current density for the improvement in resolution is limited to such a range as to allow the service lives of these elements to be secured. Thus, a sufficient resolution has not been realized to date.
It is an object of the present invention to provide a field emission electron source that can prevent a lowering of current even in a long-term operation at a high current density and enables a stable operation even at a high current density.
A field emission electron source according to the present invention includes a field emission array portion composed of an insulation layer with a plurality of apertures, which is formed on a substrate, an extraction electrode formed on the insulation layer, and a plurality of cathodes formed respectively on the substrate in the plurality of apertures. The field emission electron source further includes a cathode base for fixing the field emission array portion, and an electron lens portion composed of a plurality of electrode members having a function of accelerating and converging an electron beam emitted from the field emission array portion. In order to solve the aforementioned problem, an emission axis of the electron beam emitted from the field emission array portion has a predetermined angle with respect to an optical axis of the electron lens portion.
According to this configuration, the emission axis of an electron beam emitted from a field emission electron source is set to be shifted from the optical axis of the electron lens portion, so that the field emission electron source can be protected from ion impact caused by ions generated mainly in the electron lens portion. Thus, this configuration is suitable for improving the life of the field emission electron source.
The above-mentioned field emission electron source can be configured in the following manner. That is, in the field emission electron source, a cathode fixing surface inclined at the predetermined angle with respect to a surface of the cathode base is provided on a portion of the surface of the cathode base, and the field emission array portion is attached on the cathode fixing surface. According to this configuration, a predetermined off-axis angle can be set by using an inclined surface formed on the cathode base, thereby allowing an off-axis system to be set with high accuracy so as to have an arbitrary off-axis angle.
Preferably, a positioning mark is provided in either or both of the surface of the cathode base and a surface of the substrate. According to this configuration, the positioning of the substrate relative to the cathode base can be performed with high accuracy, and the influence of beam distortion caused by a positioning error can be reduced, thereby contributing more to the improvement in resolution.
Furthermore, preferably, at least a part of the electron lens portion has a function of deflecting an angle of the emission axis of the electron beam emitted from the field emission array portion so that the angle of the emission axis coincides with the optical axis of the electron lens portion. According to this configuration, the optical axis can be adjusted by applying a voltage to the electron lens portion, thereby allowing an electron lens to be designed more freely.
Furthermore, preferably, an electron emitting region of the field emission array portion is disposed in a shielding region that faces a non-aperture region of a first electrode member among the plurality of electrode members constituting the electron lens portion. The first electrode member is arranged closest to the field emission array portion. According to this configuration, the field emission electron source can be protected more completely from ion impact caused by ions generated mainly in the electron lens portion.
The above-mentioned field emission electron source can be configured in the following manner. That is, an optical axis of an electrode member among the plurality of electrode members constituting the electron lens portion has a predetermined angle with respect to the emission axis of the electron beam emitted from the field emission array portion. The electrode member is arranged closest to the field emission array portion
An electron gun can be configured using a field emission electron source of any one of the above-mentioned configurations. According to this configuration, an electron gun can be provided that can be used in a high-luminance CRT requiring a high current density.
A display device, an image display device or a cathode ray tube device can be configured by the inclusion of the above-mentioned electron gun and a unit for deflecting an electron beam emitted from the electron gun in a vacuum container. According to this configuration, the field emission electron source that can protect the field emission array portion from ion impact is used as a cathode of the electron gun, thereby allowing a cathode ray tube device having an excellent life time property to be obtained.